The US actress will be back as Martha Levinson, Lord Robert Crawley's freewheeling American mother-in-law, in the next series of the hit ITV1 period drama.

Shirley joined the cast as Cora Crawley's (Elizabeth McGovern) mother in the first part of the ITV show's third series last year, and there were rumours she was due to return.

It has also been announced that opera star Kiri Te Kanawa is joining the cast. The New Zealand-born soprano will play a house guest, and will sing during her visit.

Several other actors will appear in the upcoming fourth season of the programme, including Tom Cullen as Lord Gillingham, who is described as an old family friend of the Crawleys who visits the family as a guest for a house party, and who might be the one to mend Lady Mary Crawley's broken heart.

Nigel Harman will play a valet named Green, Harriet Walter will star as Lady Shackleton, an old friend of the Dowager Countess, and Joanna David will play a guest role as the Duchess of Yeovil.

Julian Ovenden has been cast as aristocrat Charles Blake.

Meanwhile, the producers have confirmed that villainous housemaid Sarah O'Brien won't be back. Siobhan Finneran, who played the character, is leaving the show.